Conventionally-used mixers including from a mixing console for use in a studio and to a mixer for amateurs are provided with basic operations for carrying out a mixing process by computing a weighted sum of respective input voice signals. In the case when the weighted sum is insufficient, an effects unit is coupled to an input and/or an output of the mixer so that by adding a linear or non-linear operation process thereto, target sound quality and sound field feeling can be realized. Upon mixing a musical instrument sound A with a voice B, in order to allow the voice B to be clearly heard even after the mixing process, the following methods are proposed.
A first method is to use a linear filter. More specifically, by using an equalizer that is one type of effects unit, a filtering process is carried out on an essential frequency band of the voice B so as to attenuate the same frequency band as that of the musical instrument sound A so that the hearing of the voice B is prevented from being interrupted by the musical instrument sound A. Non-Patent Document 1 has disclosed a method by which, with respect to a mixing process of a base guitar and a bass drum, a band in the vicinity of 200 Hz of the frequency band of a sound of the bass drum is attenuated so as to prevent the sound of the base guitar from being interrupted by the sound of the bass drum.
A second method is to use a processing system referred to as “ducker” described in Non-Patent Document 2. By using the ducker, for a time section in which the output level of a voice is kept at a fixed value or more, the musical instrument sound is attenuated by a fixed attenuation amount, and by overlapping the voice with the musical instrument sound after having been attenuated, these sound sources can be mixed with each other without the voice being interrupted by the musical instrument sound.
As another example similar to the second method, Patent Document 1 has described a technique relating to a sound calming process of audio sound source in which in the case when during reproduction of a music by a car stereo system, an attempt is made to make a voice from a navigation device more conspicuous only to the driver, the music that is being reproduced is subjected to a sound calming process and the navigation voice is mixed with the sound source subjected to the sound calming process.